The advent of a new round of stringent emissions legislation in Europe and North America is driving the implementation of new exhaust after-treatment systems, particularly for lean-burn technologies such as compression-ignition (diesel) engines, and stratified-charge spark-ignited engines (usually with direct injection) that are operating under lean and ultra-lean conditions. Lean-burn engines exhibit high levels of nitrogen oxide (NOx) emissions that are difficult to treat in oxygen-rich exhaust environments characteristic of lean-burn combustion. Exhaust after-treatment technologies are currently being developed that will treat NOx under these conditions. One of these technologies comprises a catalyst that facilitates the reactions of ammonia (NH3) with the exhaust nitrogen oxides (NOx) to produce nitrogen (N2) and water (H2O). This technology is referred to as Selective Catalytic Reduction (SCR).
Ammonia is difficult to handle in its pure form in the automotive environment. Therefore, it is customary with these systems to use a liquid aqueous urea solution, typically at a 32% concentration of urea solution (CO (NH2)2). The solution is referred to as AUS-32, and is also known under its commercial name of AdBlue. The urea solution is delivered to the hot exhaust stream and is transformed into ammonia in the exhaust after undergoing thermolysis, or thermal decomposition, into ammonia and isocyanic acid (HNCO). The isocyanic acid then undergoes a hydrolysis with the water present in the exhaust and is transformed into ammonia and carbon dioxide (CO2). The ammonia resulting from the thermolysis and the hydrolysis then undergoes a catalyzed reaction with the nitrogen oxides as described previously.
A reductant delivery unit is typically used to introduce the urea solution into the exhaust stream. To operate most effectively, a reductant delivery unit (RDU) requires good atomization of the urea solution being injected into the exhaust stream. Spray generation, or atomization, is created by the fluid stream breaking into droplets, while being directed in a specific direction. Breakup of the fluid stream is enhanced by keeping the fluid turbulent as it exits the RDU.
Some injectors include a plate which may have several exit apertures through which the fluid passes. If the fluid flow becomes laminar, or streamlined, to the wall of the exit aperture, the fluid droplets become elongated and create large droplets, or “ligaments,” which may be undesirable.
One way to attempt to decrease particle size has been to decrease the size of the orifice plate. As the depth or thickness of the exit aperture is minimized, atomization is improved. In addition, tolerances in the exit area of the injector must remain very tight in order to maintain turbulent flow and ensure good atomization. However, manufacturing processes provide for difficulty in achieving the injector designs for good atomization, and the welding process often results in inadequate flow streams. For example, the flow passageway can be obstructed or undesirable changed by the welding process.
Accordingly, there is a need for an orifice plate for an injector which reduces droplet size, and therefore reduces or eliminates the formation of ligaments and large droplets, thereby increasing atomization, where the plate is producible according to the desired design without manufacturing defects.